In formation of a television signal, several distinct views may be available for use at any time. In many instances, the broadcast controller may choose to present a single view. However, in some instances, it may be preferable to merge, combine and/or modify images from two or more views, using a process called chroma keying. A chroma key specifies, pixel by pixel, whether a portion of a foreground view or of a background view will be displayed. In chroma keying, a foreground portion of a first view or source may be combined with a background portion of a second view or source, by selectively replacing television signal data from the first source with data from the second source. When the foreground data from a particular source is captured, a selected key color is usually chosen for the (initial) background. A chroma key algorithm identifies that key color within the foreground data to determine where, or whether, to combine or merge the background data.
Several difficulties occur in implementation of chroma keying. The key color used as a backdrop may not have uniform luminance or chroma, due to physical defects in the display screen, imperfect lighting, the presence of other (nonuniform) lighting sources or reflections or shadows, and for other similar reasons. A relatively transparent foreground object will often exhibit the chroma key color and thus be undesirably replaced with a portion of the background image. The edges of a foreground image may manifest fine detail, such as wisps of hair, smoke, etc. that are easily lost using many chroma key algorithms. Color spill or infection from the chroma key color is often present adjacent to the edges (sharp or diffuse) of a foreground object and may arise from reflected key color light from the backdrop, blurring of an image due to local defocusing of the camera, motion of the camera and object relative to each other, video filtering, etc.
Originally, the chroma key was a “hard” key, within which only the foreground image or only the background image was displayed at each pixel. A “soft” chroma key has become available more recently, in which each pixel may include a first fraction of the foreground image and a second fraction of the background image. Use of soft keying allows a blending at an image edge and may provide a more natural transition between images.
FIG. 1 is a block diagram of a conventional chroma key system 11. A foreground image and a background image are received on signal lines 13 and 14, respectively, at a foreground suppression module 15 and at a background suppression module 17. A foreground key generator 19 and a background key generator 21 provide a foreground key and a background key for the foreground suppression module 15 and the background suppression module 17, respectively. The foreground suppression module 15 and the background suppression module 17 provide partially (or fully) suppressed foreground and background images that are received by an image sum module 23. The result is a chroma keyed image that may combine and issue the foreground and background images in a composite video image according to the particular keying algorithm adopted.
FIG. 2 is a block diagram of an existing alpha mixer system 31. A foreground video signal FG and alpha signal α(0≦α≦1) are received on input signal lines 33 and 35, respectively, and are multiplied to form a first product, α·FG, in a first multiplier module 37. A background video signal BG is received on a third input signal line 39, an α-complement, 1−α, is formed in a complement module 41, and a second product, (1−α·BG, is formed in a second multiplier module 43. A sum of the first product and the second product is formed in a sum module 45, and the resulting mixed video signal, α·FG+(1−α)·BG, is issued as an output signal on an output line 47.
What is needed is a soft chroma keying approach in which one or more foreground images can be combined with one or more shadowed background images in a programmable approach that allows the relative strength of each image to vary, pixel by pixel, with time across the display screen. Preferably, the sum of the strengths of the contributing images should not be required to add to 1.0. Preferably, the distribution of chroma key values over a color plane (a “key map”) should be expressible as a mapping between selected foreground location on the color plane and corresponding n-tuple representing the chroma key values for each pixel.